Geiger-mode avalanche photodiodes (GmAPDs), also referred to as single-photon avalanche diodes (SPADs), are designed and fabricated by our group at MIT Lincoln Laboratory. When bonded to a readout integrated circuit (ROIC), they form a system that can timestamp single photon arrival with sub nanosecond precision. When the pixels are armed in Geiger mode, they detect photons by creating an avalanche of electron-hole pairs in the detector material that can be detected by a ROIC. This paper explores a phenomenon known as afterpulsing, which can manifest itself as an increase in detector noise, or dark count rate. Afterpulsing occurs due to defects in the device structure that cause charge carriers from a previous avalanche to get trapped within the impurities of the device. If the extra charge carriers aren’t provided enough time to depopulate from the traps, the re-biasing of the individual device back into Geiger-mode operation has a time-based, statistical impact on the likelihood that the trapped carrier causes a secondary avalanche to occur upon re-arm. We investigate afterpulsing using a MIT Lincoln Laboratory designed 32x32 asynchronous readout integrated circuit bump-bonded to a InGaAs/InP 1550nm GmAPD array. This paper reports on how afterpulsing is affected by changing operating temperature, applied overbias voltage, and/or individual pixel holdoff time. Additionally, methods of determining afterpulsing with on-ROIC pixel interarrival data are discussed and best operating parameters to minimize afterpulsing for our GmAPD and ROIC are presented.

In this paper, we report the temperature stability and robust performance of our avalanche photodiode (APD), as well as the wide dynamic range and high gain linear mode detection ability of our receiver module prototype. The APDs have a low dark current and low-temperature coefficient of operating voltages from 19.2 to 22.7 mV/°C with a gain from 10 to 200. The high sensitivity APD- transimpedance amplifier (TIA) receiver module (TIA gain 22 kV/A) demonstrates measured NEP value of 29 fW/Hz^0.5 when APD operates @M=130 under room temperature and predicted NEP value of 18 fW/Hz^0.5 @M=200 under 0°C for the 180 MHz of measurement bandwidth. This corresponds to a detection level of 16 and 10 photons respectively. The NEP value under 85°C is predicted as 77 fW/Hz^0.5 when APD operates @M=60. This receiver module has a fast overload recovery of 1.39 μs under 51 kW/cm² optical power illumination with 1045 kV/W of overall responsivity for APD-TIA. Our APDs also show robust performance of optically induced damage threshold of 40 MW/cm² optical power illumination under circuitry protection and power dissipation limit of ~190 mW without circuitry protection.

Superconducting nanowire single-photon detectors (SNSPDs) have made a tremendous impact in recent years on a diverse array of research fields, by enabling researchers to access near unity system detection efficiency, fast count rates, and precise timing through commercial availability. Originally focused on discrete devices operating between the Near IR to Telecom wavelength regimes, the industry is expanding to the visible and mid-infrared domains while incorporating multi-detector structures and photon number resolving techniques. This talk will discuss developments at Quantum Opus in these areas as well as emerging applications in new fields for SNSPDs.

We present our progress in the realization of a waveguide-integrated, near-infrared, 20-channel pseudo photon number resolving architecture realized on the silicon nitride platform. Each channel consists of a space multiplexing network directing to 8 superconducting nanowire single-photon detectors connected in series. Our design focusses on minimizing splitting losses, ensuring robust response across different wavelengths in the near-infrared range and achieving low noise electrical readout. Specifically, we will report on the design of wavelength-independent splitting elements operating in the near-infrared, the realization of low-loss fiber-to-chip coupling and detail the implementation of the cryogenic biasing and readout scheme. Our work constitutes a significant advancement towards practical multi-channel photon number resolving detectors, serving as a promising building block for scalable and high-performance quantum information processing systems.

State readout of trapped-ion qubits is usually achieved by observing qubit-state-dependent fluorescence from the ion while driving an optical cycling transition with laser light. The integration of photon detectors for fluorescence detection into the ion trap itself may benefit the development of many-qubit setups for applications like quantum computing. Superconducting nanowire single-photon detectors (SNSPDs) are promising candidates for trap-integrated detectors for high-fidelity trapped-ion qubit state readout. However, the strong oscillating electromagnetic fields that are typically used to trap and manipulate ions can affect the function of the SNSPDs significantly. In this work, we demonstrate an improved design to integrate SNSPDs into linear rf ion traps that reduces the susceptibility of the SNSPDs to applied rf trapping potentials. Our measurement results represent an improvement in rf tolerance by an order of magnitude with an increase in operation temperature from 3.5K to 6K compared to previous work.

The usage of a GaAsP (gallium arsenide phosphide) photomultiplier for microscopical imaging allows the evaluation of low-light luminescent objects. We designed a setup for collecting a confocal microscopic image signal, which is divided into 14 equal-sized input channels. The division is achieved with a beamsplitter and two fiber bundles consisting of seven fibers each. Re-imaging the confocal pinhole by such a densely packed fiber bundle permits the utilization of a photon re-localization approach to overcome the optical resolution limit. The center fiber creates a real-time image, while the outer fibers enable a higher-resolution image via an image scanning microscope (ISM) signal calculation. The fiber bundles are enclosed in a fused silica capillary and are drawn out to create one solid fiber bundle. During the drawing process, the fiber bundles are tapered down to an outer diameter size of 400μm, with each fiber having a less than 0.3 Airy unit diameter. For the photomultiplier interface, all fibers of both fiber bundles are integrated into a v-groove array, with each fiber representing a detection input, which is followed by projection optics for imaging onto the multichannel detector. The resulting confocal super-resolution microscope is suitable for the application of time-correlated single photon counting (TCSPC) techniques such as fluorescence lifetime imaging (FLIM), time-resolved anisotropy, or F¨orster resonance energy transfer (FRET) imaging.

Motivated by the need for higher data volume return from NASA’s future planetary missions, the deep space optical communications (DSOC) technology demonstration (TD) was initiated. The DSOC project integrated a flight laser transceiver to the Psyche Mission spacecraft which launched on Oct. 13, 2023. Ground transmitting and receiving stations implemented by instrumenting existing assets, make more or less weekly optical links with the receding spacecraft. Compared to past and ongoing free-space optical communication (FSOC) TD’s, the key difference for DSOC, is the huge increase in link distance. Photon-counting at both ends of the link were mandated to overcome the link difficulty, defined as data-rate times link-distance squared. In this paper a summary of the photon-counting applications and experience gained so far will be described.

We describe a method for reducing the temperature sensitivity for spontaneous parametric down conversion (SPDC) suitable for generating entangled time-frequency quantum photonic states in the telecommunication band at a wavelength of 1550 nm. We present specific design examples using periodically-poled LiNbO3 waveguides, which are commercially available and have a high second-order nonlinear coefficient that improves the overall device efficiency. One method uses two distinct poling regions, each with a poling period selected for phase-matching at different temperatures. The temperature bandwidth is increased from ~5 °C to ~17 °C using this approach, while only reducing the overall efficiency of the process by a factor of 4. The bandwidth is large enough so that controlling the overall temperature of the nonlinear crystal is likely not needed in a laboratory environment or can be achieved using a low size, weight, and power temperature controller design for field-deployed sources. To improve the stability of the design, we propose using a differential heating method where only the difference in the temperature is stabilized. The differential temperature stabilization method requires a smaller, lighter weight heater and lower power than other approaches, thus greatly simplifying the overall design. Additionally, we show that the system is less sensitive to fluctuations in the pump laser’s wavelength for the two-region approach. Finally, we generalize the design to multiple poling regions.

Quantum memories play a pivotal role in establishing long-distance communication by entanglement swapping operations within quantum repeater nodes. Constructing such a quantum memory involves Electromagnetically Induced Transparency (EIT) within atomic vapors at room temperatures to store highly attenuated coherent light pulses down to the level of single photons. The photons may be generated from quantum nodes containing stationary quantum systems, such as atoms or semiconductor quantum dots (QDs). QDs serve as a potent source of quantum light, furnishing bright, precisely timed single photons of exceptional purity. While previous endeavors have demonstrated the integration of QDs with atomic vapors through techniques like “slow light,” the development of a dedicated quantum memory for QDs remains unmatched. In our study, we introduce an EIT quantum memory hosted within warm cesium vapor. Our approach exhibits a good efficiency in storing faint coherent light pulses at the single photon level. Moreover, the measured bandwidth of around 200 MHz approaches the Fourier-limited emission characteristics of QDs. We present initial efforts to match the emission from QDs with our quantum memory and discuss application scenarios of room temperature EIT quantum memories.

Quantum sensing and quantum communication systems rely on high-performance single- or entangled-photon sources and single-photon detectors enabling experiments based on the quantum nature of single photons. In this contribution, we discuss the development of an entangled-photon source delivering entangled photon pairs with wavelengths of about 1550 nm alongside with single-photon avalanche detectors (SPADs) for the short-wave infrared (SWIR) and for the extended SWIR (eSWIR) spectral range. The fabrication processes of such quantum-enabling technologies is highlighted. The entangled-photon source is based on AlGaAs Bragg-reflection waveguides. Very low difference in effective refractive index of TE and TM polarized photons – important for high polarization entanglement without external compensation – as well as high single and coincidence count rates were achieved. For the fabrication of InGaAs/InP SWIR SPADs, the key technology is the planar process technology via zinc diffusion to produce spatially confined p-type regions. For the zinc-diffusion process, a novel method of selective epitaxial overgrowth was developed, achieving the intended double-well diffusion profile. Experimental data of thus fabricated InGaAs/InP SPADs show the expected dark-current, photo-current, and multiplication-gain characteristics in linear-mode operation as well as breakthrough behavior in Geiger-mode operation at 240 K, which is a typical operating temperature for InGaAs/InP SPADs achievable by thermoelectric cooling. GaSb-based SPADs for the eSWIR are fabricated in a mesa approach showing the expected dark current behavior as well. All three different devices are linked by enabling quantum technologies in the (e)SWIR as well as by using our III/V-semiconductor technology facilities.

In this paper, we review the research and development of the fractal superconducting nanowire single-photon detectors (SNSPDs), including our demonstrations of high-performance devices and systems with over 80% system detection efficiency, negligibly low residual polarization sensitivity, and low timing jitter. Using the fractal SNSPDs, we demonstrate full-Stokes polarimetric imaging LiDAR.